1. Field of the Invention
This invention relates to electrical connections and will be described in relation to the use of such connections with leads for cardiac pacemakers. However, it is to be understood that the invention is not limited to pacemaker electrode leads.
2. The Prior Art
Conventional unipolar pacemaker systems consist of a pulse generator and a unipolar pacemaker electrode. In such systems, the return path or indifferent electrode includes the whole or at least a part of the metallic pacemaker case. The drawback of such an arrangement is that the return path of a sensed signal is via tissue fluid and thus subject to interfering signals such as myopotentials. Additionally, electrical activity at the indifferent electrode may give rise to undesirable muscle tissue stimulation or twitching. For these reasons, unipolar pacemakers are usually coated with an insulating layer (such as silicone rubber or paralyne), leaving exposed only a small metal area which is disposed so as to face fatty tissue and thereby act as the indifferent electrode.
A bipolar pacemaker system consists of a bipolar pacemaker connected to a bipolar pacing electrode lead. The bipolar lead has an indifferent electrode incorporated in it which reduces the distance the signal has to travel and minimizes signal interference. In addition, the bipolar pacemaker case is electrically neutral, thus eliminating the problems of muscle twitch and the need for pacemaker coating. These advantages often make the bipolar system the design of choice in pacemaker implants.
A bipolar pacing lead typically consists of a distal electrode tip, commonly made of platinum or platinum/iridium, and an indifferent ring electrode located a specified distance away from the distal tip. Known bipolar pacing leads have welded or swaged electrode ring constructions, such as that disclosed in U.S. Pat. No. 4,328,812, but which results in a discontinuity in the insulation at the site of the ring. This discontinuity necessitates a relatively complex design at the ring electrode region in order to maintain a seal against ingress of body fluids. The sealing arrangement around the ring, together with the necessity of having a metal supporting insert in the case of a swaged construction, results in a bulky ring assembly and a significantly enlarged lead cross section.
The enlarged ring section makes passage of the lead through the veins of a patient difficult and can also result in thrombus formation. Welded or swaged designs may cause damage to the ring electrode surface during construction thereof, which also increases the possibility of thrombus formation. Welded and particularly swaged electrode ring assemblies also reduce lead flexibility in the area immediately adjacent to the ring.